All carcasses of pneumatic green tires are built as a series of layers of flexible high modulus cords encased in a low modulus rubber; the cords in each layer are oriented in a chosen path or direction and substantially equispaced and parallel. The tire, whether belted radial ply or bias ply, is cured in a curing press using a curing bladder which forces expansion of the tire. When the carcass is cured with an innerliner in it, as it usually is, the innerliner expands with the carcass which is forced against the indentations in the curing mold to form the tread, and all components are co-cured so as to provide a substantially cohesive bond between one and another.
Expansion upon curing of a radial ply tire is small, in the range from about 2% to 20% greater than the size of the green carcass; but expansion of a bias ply tire may range from 20% to 250% or more. Since, at present, a release agent is already provided on the surface of the curing bladder, and/or on the inner surface of an innerliner, using a self-supporting film of solid polymer to function as an additional release agent is contraindicated because it is unnecessary.
An innerliner for a pneumatic tire is typically formed from a compound containing a major proportion by weight of a halobutyl rubber. Before the tire is cured, the entire original inner surface of the innerliner and/or the outer surface of a shaping bladder used in the curing press, is coated with a release agent. The release agent is commonly referred to as a “lining cement” when used on the surface of the innerliner; and, to a “bladder lube” or “bladder spray” when used on the shaping bladder. Some tires are cured without an innerliner because it will be applied, typically by spraying, after the tire is cured.
In this invention, the tire carcass is provided with an adherent, removable, self-supporting solid barrier film or layer of specific synthetic resinous compounds, which film protects the original interior rubber surface of the carcass under the film, or the tread under the film, or the sidewalls under the film, from contamination by release agent.
The surface of the innerliner, or the interior surface of an innerliner-free green carcass, or the exterior surface of the tread, or the exterior surface of the sidewall, protected so as never to have come in contact with release agent, is referred to as a “virgin surface” whether it is cured or not. Such a virgin surface permits a rubbery article to be bonded to it without having to clean the surface; at the present time, such bonding to a contaminated surface is typically done after cleaning it, first by buffing with a wire brush in combination with an appropriate solvent, followed by vacuuming the solvent. Such cleaning is necessary to remove the lining cement or bladder spray (release agent), typically an organopolysiloxane—or “silicone”—based material such as poly(dimethylsiloxane) including powdered mica or crystalline silica and afford a “cleaned surface”. Cleaning is both time-consuming and environmentally unfriendly, since the solvent is non-aqueous and aggressive; moreover, its use is restricted. Nevertheless, before automobile tires are provided with a puncture sealant, applied to the inner surface of a cured innerliner, it must be thoroughly cleaned. Cleaning to get rid of contaminating release agent so as to provide a “cleaned surface”, has been done over several decades, and is still done.
Alternatively, and less preferably, the release agent may be removed by washing with an appropriate detergent, or mechanically, by buffing or abrading the surface until the contaminant is removed. Since tire manufacturers were enured to the disadvantages and additional cost of a cleaning step, they were unaware that bonding a rubbery tire component to a cured virgin surface of a tire provides an unexpectedly stronger bond than bonding to a “cleaned surface”, even if it is meticulously buffed and solvent-cleaned.
Even a small aircraft tire 450-190-5 (450 mm diameter, 190 mm width and 5″ or 127 mm bead diam.) of bias ply construction typically requires an average of about 235 ml of an environmentally unfriendly solvent; at present, solution of the problem appeared to mandate finding a less objectionable solvent.
A carcass of a pneumatic rubber tire, whether a radial or bias ply, is often required to have a rubbery component bonded to a portion of the tire's surface, either exteriorly on the sidewall, or internally within the toroid. For example, an aircraft tire, typically of bias ply construction, is dynamically balanced by adhering a laminar pad of rubbery material, referred to as a “balance pad”, symmetrically about the circumferential centerline of the interior surface of the cured tire. However, because the precise position at which the balance pad is to be adhesively secured, cannot be determined until the tire is cured and dynamically balanced, the midportion of the entire inner liner is cleaned.
A balance pad, such as one commercially available from Patch Rubber Company, is a multilayer rubbery component which typically includes (i) a thick layer of high specific gravity compound blended with iron oxide and cured, the thickness being a function of the weight desired, (ii) a relatively thin layer of high elongation floater gum or stretch ply of rubber filled with carbon black, (iii) a bonding gum layer (also referred to as a “gray-face gum” layer) of curable rubber compound with curing agent but without a cure accelerator or activator fluid, and (iv) a protective film covering the bonding gum layer. When the protective layer is removed from a balance pad of desired weight, and the exposed bonding gum layer is secured with a fast-dry cement containing a cure-accelerator, typically an alkylamine or aralkylamine, to the rubbery surface of a cured innerliner, or of an innerliner-free cured carcass, the curing of the bonding gum layer to the rubbery surface, typically at ambient temperature over a period of several days, ensures that the balance pad will not be dislodged during operation of the tire. However, since one cannot know in advance where the balance pad will need to be positioned, the entire surface of the innerliner must be clean.
Another example of a cured tire requiring a cleaned surface is that of a “smart” tire in which an electronic monitoring device is to be secured. Such a device is used to record the operating history of a tire including temperature and pressure, distance travelled, impacts sustained, and other data, to transmit the data to the driver or to a designated receiving station, and to do so without taking the tire out of service. A typical monitoring device includes a passive integrated circuit and antenna encapsulated and non-removably secured within a tire, the device being activated by a radio frequency transmission that energizes the circuit by inductive magnetic coupling. Such devices of numerous configurations and types currently being used have a common requirement, namely, that each be so securely mounted within the tire that it will tolerate any condition to which the tire is subjected without being dislodged. The problem is that flexure of the tire, and particularly of its tread and sidewall, results in dislodging the device.
This problem has been addressed in U.S. Pat. No. 6,244,104 to Koch et al by supplying a patch having several layers; the uncontaminated layer to which the monitoring device is to be secured is not the virgin surface of the innerliner. They provide a protective flexible layer of Mylar® polyethylene terephthalate film, plastic, metal foil, metal screen or a polyurethane over a cure cloth (or cure paper) to prevent an uncured rubber layer from curing into the cured rubber layer. To provide temporary protection against contamination of the uncured rubber layer before the patch is placed on a desired are and co-cured with the tire, a protective film of polyethylene (“PE”) is used.
Koch et al recognized that, to ensure a cured inner surface free from trace amounts of remnants of a release agent, by far a better choice than cleaning the contaminated surface, with or without a solvent, was to provide an additional uncured rubber layer protected from contamination in a patch. The patch provided a newly added uncontaminated cured rubber surface over the virgin surface of the innerliner directly cured to the protective patch with its curable rubber layer. Koch et al do not “protect” any portion of the surface of the innerliner before it was cured; they provided an additional cured surface superimposed upon the cured innerliner.
To protect the entire virgin surface of either a portion of the exterior surface of the tire, or an innerliner, logic dictated that the virgin surface be protected from a conventional release agent in the first place, and that this be done by a removable barrier film between the curing bladder and the virgin surface while the tire was being cured, the barrier film to be readily removable after curing.
The Problem
The problem is implementing the logical choice, that is, to counter contamination of the entire innerliner surface by the already-present release agent at a temperature in the range at which the tire is to be cured; in some instances only a minor portion of the innerliner may need to be protected. Choice of a barrier film dictated that it be heat-resistant in that temperature range, typically from about 121° C. (250° F.) to 200° C. (392° F.).
Further, expansion of a green carcass in the curing press dictates that, to protect a major portion of the innerliner's surface, the barrier film be extensible at least 2% at curing temperature in any direction on the surface the barrier film is to protect, and to stretch during curing without tearing. A green belted radial ply tire for an automobile expands in the curing press in a range from about 1% to 20%; a conventional green cross bias casing of a bias ply tire with a crown angle in the range from 20° to 38° expands in the curing press in a range from about 20% to 250%, expansion of aircraft tires being greatest. Therefore a usable barrier film is required to be adequately expandable within the curing carcass, that is, multiaxially expandable, without tearing in the range from about 5% to 100%. The barrier film is also required to be adequately thermoformable, in that it conforms to the shape of the bladder during curing, thus squeezing out entrapped air, and after being thermoformed the film substantially retains its formed shape as the film has essentially no memory and is non-elastomeric. Still further, to be manually readily removable, it is essential that the barrier film not fuse to itself when overlapped and heated. In the instance when the entire virgin surface is to be protected, one end of the barrier is overlapped over the other (which other end is applied or “stitched” to the virgin surface to be protected) so as to form a “pull-tab” for easy removal.
Efficiently curing a tire requires that heat transfer from the bladder be attenuated as little as possible; this mandates using a relatively thin film of polymeric material. Though it is possible to calender natural and synthetic hydrocarbon rubbers to as thin as 0.305 mm (0.012″ or 12 mils), then partially or fully pre-cure them for use as barrier films, they do not lend themselves to being reliably calendered into a sheet less than 0.762 mm (0.03″ or 30 mils) thick. When so calendered and essentially fully cured, the cured rubber is difficult to stitch into the green innerliner, and fails reliably to withstand the expansion of a curing bladder, first at ambient or relatively low temperature, then in a hot mold, without tearing. When partially precured and having some tack, for easy positioning, the cured rubber strip is not readily integrally removed; that is, in a single piece without tearing into two or more pieces as it is being removed.
Since it is self-evident that the barrier film is to be positioned either on, or within the green tire, and that its position be maintained until it is loaded into the curing press, the requirement that the virgin surface be maintained as such, dictates that no adhesive be used to position the barrier film within the green carcass, or on the exterior surface of its sidewall; and, no release agent, solid or liquid, remain on the tire liner's inner surface when the barrier film is removed.
Still further, since a substantial period of time may elapse before the cured tire is taken up in a production line to have a desired component adhesively secured to it, it is desirable that, before the tire is cured, the barrier film be secured to the virgin surface of the cured tire, whether it has an innerliner or not, sufficiently well that only a small force in the range from about 0.4 to 7.9 N/cm (1 to 20 N/inch) is required to remove the barrier film. Moreover, it is essential that, after the tire is cured, the barrier film remain on the innerliner and not fall off into the tire mold.
Part of the foregoing problem was addressed in U.S. Pat. No. 6,217,683 to Balzer et al taught a plastic or rubber sheet applied directly to a surface area on the inner surface of the tire in a bead area on either side of the tire prior to curing the tire. The plastic or rubber sheet has a smooth surface which provides a generally smooth area when forced into the inner surface of the tire during curing of the tire. Smooth sheet can be made using any suitable compound impermeable to silicon (sic). The sheet is removed after a given post-cure time to provide a conditioned, treated, clean and smooth surface area within the tire suitable for affixing the rubber ply to surface area. (see col 13, lines 38-49).
To begin with, a rubber sheet is inoperable as it cannot be removed from the innerliner after the tire is cured, as seen from the evidence relating to the Sandstrom U.S. Pat. No. 4,443,279, presented below. A usable “plastic” sheet must be thermally stable at curing temperature; if it is embedded in the cured virgin surface it is difficult to remove; if the sheet is not embedded either in the exterior surface of the carcass or in the innerliner, the sheet is likely to fall off in the mold when the tire is being removed. The disclosure of a “plastic sheet” is not an enabling disclosure.
Substantially the same problem was addressed in U.S. Pat. No. 4,443,279 issued to Sandstrom, who provided a barrier strip having a thickness in the range of about 0.025 to about 0.25 cm, the strip containing (a) 60 to 90 parts by weight uncured butyl rubber and correspondingly, (b) 40 to 10 parts by weight of an ethylene/propylene/nonconjugated diene terpolymer (EPDM) which contained 2 to 10 parts by weight of a tackifier resin; to this mixture was added conventional curing compounds. The co-cured strip is stated to have then been removed. Though, whether the integrity of the co-cured strip was maintained when it was removed, is unstated in the '279 patent, the data presented below indicate that when the illustrative example was duplicated the strip was not readily removable; it tore when it was being removed. Moreover, though the '279 strip is desirably stated to have a relatively low adhesion to the inside surface of a cured tire of less than about 1.8 Kg/linear cm (10 lb per linear inch), the data presented below indicate that the illustrative example does not meet the requisite specification for removability. Moreover, when one end of the cured strip overlaps the other, the ends become fused together upon curing and cannot form a pull-tab for manual removal. Further, since Sandstrom coated the interior of the cured tire with puncture sealant after the co-cured strip was removed, he did not have reason to compare the strength of bonds obtained between the surface of a rubbery article and conventionally cleaned and virgin surfaces respectively, irrespective of how the latter may be provided; he missed finding the advantage of maintaining a virgin surface.
One skilled in the art will know that, with the exception of a waxy release paper, films of numerous synthetic resinous compounds such as Mylar® polyester, Saran® vinyl chloride-co-vinylidene chloride, cellophane, polyurethane and polyolefins such as polyethylene (PE) and polypropylene (PP), can be “stitched” with varying degrees of success, onto the exterior of, or into the interior of a green tire because the uncured rubber is tacky enough to do so. Even a heavily cured (high cross-link density) strip of rubber may be stitched into, and remains positioned in the interior, though not reliably; and upon curing, the strip is readily removable, but it too-often tears in the mold because it does not expand sufficiently, and is usually removed in pieces; having been rent, it fails to protect the virgin surface from contamination by the bladder lube coated on the curing bladder. Further, if the curing bladder is not coated, it will adhere to the portions of the carcass where tears in the cured strip have occurred, damaging the bladder when the carcass is torn from it.
Even substituting a cured thin first strip for the uncured strip used by Sandstrom, fails to provide an effective barrier layer because the pre-cured strip tears upon removal. Substituting a less heavily cured (lower cross-link density) second strip which will not tear (and is more readily stitched into the interior of the green carcass than the cured), provides the necessary expansion and excellent protection when the tire is cured—but the second strip still adheres to the protected surface too tightly to be removed integrally, and cannot be easily removed.
An expectation that tailoring the composition of the green uncured strip with the “right” amount of curing compounds would yield, without undue experimentation, a readily removable strip which would not tear upon removal, was justified in view of the known difficulty of tightly adhering a rubbery surface to the cured surface of a butyl rubber innerliner having a conventional composition of isobutylene-based polymers, predominantly brominated butyl rubber optionally blended with one or more other rubbers. Such justification was unfounded.
As evidenced by the results presented below, a barrier film chosen from readily available films of precured and cured rubber, Mylar, Saran, polyurethane, cellophane, PE and PP was ineffective. Though thermally stable at curing temperature, Mylar and cellophane films wrinkle in the mold because they do not expand, and lining cement enters underneath. They are effective only when a portion of the innerliner is to be protected, provided the film stays in position. In practice, the film becomes dislodged and falls off in more than 10% of cured samples which is unacceptable. Saran®, PE and PP are thermally unstable. Hydrogen chloride generated by decomposition of Saran contaminates the mold. PE and PP films no thicker than 127 μm, 0.127 mm (5 mils) melt and fuse with the lining cement which contaminates the innerliner. A polyurethane strip less than 5 mils thick is too rubbery to pull out off the tire. If one end of a PE or PP strip, thick enough not to disintegrate, overlaps the other end in a curing tire, the overlapping end becomes fused to the strip and cannot provide a pull-tab to try and remove such portion of the strip which does not disintegrate.